This invention relates to a printing unit, in particular for a rotary offset printing press with flying plate change capacity.
Rotary offset printing presses, used for example in the printing of newspapers, including colour printing on each side of a substrate such as paper generally comprise a plurality of printing units.
A printing unit prints one colour on each side of a substrate, usually paper. In the offset lithographic printing process for double-sided printing, or perfecting, the printing unit comprises a pair of printing couples. Each couple comprises a plate cylinder and a blanket cylinder. Each plate cylinder carries one or more printing plates around its periphery. Each printing plate has portions which are water sensitive and portions which are not. Ink and a dampening solution, such as water, are applied to the plates on the printing cylinder. The presence of the dampening solution on the water sensitive portions means that the ink only remains on the non-sensitive portions. Each plate cylinder is pressed against its co-acting blanket cylinder which is a cylinder with a resilient surface, usually an elastomeric material. As the cylinders in each couple rotate the ink image formed on the plate cylinder is transferred to the blanket cylinder. The substrate, eg. newsprint, passes between the two blanket cylinders of the printing unit which transfer the ink image onto each side of the substrate.
In the printing industry, in particular the newspaper industry, there is a need to be able to change any page content, particularly on the front and back pages, with the minimum of printing machine down time.
One proposal is disclosed in U.S. Pat. No. 5,134,934 The proposal involves adding an additional plate cylinder to each printing-couple such that each blanket cylinder has two associated plate cylinders. The blanket cylinders have a first and a second printing position such that each blanket cylinder is in contact with a respective first or second plate cylinder and is also in printing contact with the substrate. There is an intermediate off-printing position in which the blanket cylinders are neither in contact with printing cylinders nor the substrate.
In this proposal, the blanket cylinder axes are moved by means of eccentric bearing sleeves where the axes of the bearing sleeves are closer to the substrate path than the blanket cylinder axes, such that the blanket cylinders move in a direction substantially parallel to the substrate path between the first and second printing positions. With the blanket cylinders in the first or second printing position the plate cylinders on either side of the substrate not in contact can be stopped and their plates changed while the press is still operating.
This apparatus has the drawback that each of the two blanket cylinders-on either side of the substrate must be moved together in order to maintain printing geometry. Thus if it is desired to change the plates on one side only, for example for changes to the front or back page as is often the case in the newspaper industry, for the side that is not being changed either a duplicate set of plates must be provided with the obvious wastage and extra labour required to fit the plates or the plates for that side must be swapped between the plate cylinders which is time consuming and greatly reduces the advantages of on the fly plate changes. It is also essential that each blanket cylinder is provided with two plate cylinders whereas it might be desired in some printing units only to change the plates for one blanket cylinder and thus provide a single plate cylinder for the other blanket cylinder. Furthermore, as the blanket cylinders must be moved substantially parallel to the moving substrate and also release and then regrip the substrate while the substrate remains in motion, there is a risk of tearing of the substrate.
DE-U-84 10 619 discloses a printing unit for printing on one side of the substrate wherein a blanket cylinder is associated with two plate cylinders. Each plate cylinder is arranged to be movable in a direction parallel to the path of the substrate to bring the plate cylinder into or out of contact with a fixed blanket cylinder. The disclosure does not address the problems with such a system, in particular how the plate cylinders are arranged for movement whilst ensuring that the plate cylinder will maintain register for further printing, nor how, in particular with printing units arranged on either side of the substrate for printing on each side, the blanket cylinders may be moved apart to release the substrate, as will be required from time to time.
It is an object of the present invention to alleviate, at least partially, some of the above problems.
Accordingly, the present invention provides a printing unit for double-sided offset printing on a substrate, said unit comprising:
a pair of blanket cylinders disposed one on either side of a substrate path;
a first pair of plate cylinders each selectively moveable into and out of contact with a first one of said pair of blanket cylinders;
wherein at least one of said blanket cylinders is moveable to a first position for pressing said substrate against the other blanket cylinder and is moveable to a second position to provide a clearance between said blanket cylinders along said substrate path.